DEVELOPMENT OF ATRIAL CHAMBEE OE AMPHIOXUS. 457 



Each larva is illustrated by three views : one a surface- 

 view of the left side complete, one a surface-view of the right 

 side of the anterior end, and one a deep focus of the anterior 

 end. 



The drawings were made from carefully preserved specimens 

 (killed with corrosive sublimate), stained with carmine and 

 mounted in balsam. They are diagrammatic in the sense that 

 they represent the results of observation rather than an actual 

 view as obtained by one focussing. 



The most striking feature in both larvse is the large mouth 

 on the left side. In the younger larva the form of the tail, 

 with its peculiar larval fin-rays, is noteworthy. In many 

 respects this larval tail-fin recalls that of young Teleostean 

 fishes. It is also closely similar to that of some Ascidian 

 tadpoles (e. g. Styela). The small number and large size of 

 the myotomes (indicated by numbers in the drawings) in the 

 anterior region of the body are also remarkable. No evidence 

 could be obtained by us of the intercalation of new myotomes, 

 nor of the multiplication of anterior myotomes by division. 



The new myotomes appear to form exclusively at the caudal 

 extremity. 



In the larger larva the full number of adult myotomes has 

 been attained, and the larval tail-fiu has become greatly modi- 

 fied, giving place to the mesoblastic expansion which forms the 

 tail-fin of the adult. 



When we remember that in the adult the oral sphincter lies 

 in the vertical line of the apex of the tenth myotome, it is not 

 a little astonishing to note the position of that myotome rela- 

 tively to the alimentary canal in the younger larva, and even in 

 that which has attained the full complement of myotomes. The 

 independence of the metamerism of the body-wall from that 

 of the gill-slits and alimentary canal is thus very sharply 

 indicated. 



In the cephalic region of both the older and the younger 

 larva we see two remarkable larval structures, which lie in 

 front of the mouth — the one in front of the buccal cavity, and 

 the other within its area. These are the prreoral pit and the 



